<template>
 <ul class="list">
     <li class="item" 
        v-for="item in letters" 
        @click="handleLetterClick"
        @touchstart="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"
        :key="item"
        :ref="item">{{item}}</li>
 </ul>
</template>

<script type="text/ecmascript-6">
export default {
    data () {
       return {
           // 标识位
           touchStatus:false,
           startY:'',
           // 函数节流
           timeer:null
        }
    },
    // 父组件传数据让子组件更新后触发
    updated(){
        this.startY = this.$refs['A'][0].offsetTop
    },
    computed:{
        // a-z字母数组
        letters(){
            const letters = [];
            for(let i in this.cities){
                letters.push(i)
            }
            return letters
        }
    },
    props: {
        cities:Object
    },
    components: {

    },
    methods:{
        handleLetterClick(e){
            this.$emit('change',e.target.innerText)
            console.log(e.target.innerText)
        },
        handleTouchStart(){
            this.touchStatus = true;
        },
        handleTouchMove(e){
            if(this.touchStatus){
                if(this.timer){
                    clearTimeout(this.timer)
                }
                this.timer = setTimeout(() => {
                    const touchY = e.touches[0].clientY - 79
                    const index = Math.floor((touchY - this.startY)/20)
                    if(index >= 0 && index < this.letters.length){
                        this.$emit('change',this.letters[index])
                    }
                }, 16);
               
            }
        },
        handleTouchEnd(){
           this.touchStatus = false; 
        }
    }
}
</script>

<style lang="stylus" scoped>
    @import "~styles/varibles.styl"
    .list
        display:flex
        flex-direction:column
        justify-content :center
        position:absolute
        top:1.58rem
        right:0
        bottom:0
        width:.4rem
        .item
            text-align :center
            line-height :.4rem
            color :$bgColor

 
</style>
